Randy Bush writes:
Not sure what you mean here concerning 'unroutable' prefixes, but the issue with obtaining an allocation for one of the upstream provider's CIDR block when multihomed *does* have its drawbacks, at least from the end-user perspective. If said prefix (let's say a /24) is announced in the 'allocating' provider's aggregate, and the more specific is announced via the 'other' provider, the more specific will always be preferred.
This is brain damaged. Given
AS1 ----- Sprint | | | | AS2 ----- anything else not Sprint
You can not announce a bit of Sprint space AS1->AS2->MCI as a fallback (note the 'extra' AS hop) because Sprint aggregates your announcement and the longer prefix is announced to the world via <anything else>.
Use Sprint space, bye bye fallback.
To the best of my knowledge (which ain't that hot), all other providers have discovered suppress-map.
This is mainly due to the fact that Sprint does not listen to any announcements from its peers for anything within its "non-portable" blocks. Multihoming and expecting fallback to work are two different things I guess. Let's not even mention the AS path length stuff. BTW, Sprint also does not listen to any of its peers ASes through other peers so peering with Sprint and still paying someone for transit services do not help you get more redundancy with Sprint's network. -Hank